Probably been suggested before, but worth putting forward again - Perth to Inverness (or part thereof) in the classic diesel era. If you think about it much of the required traction and rolling stock already exists - Class 40, Class 47 and Mark One/early Mark Two stock. Would need (maybe) some Type 2 Diesels - Class 24 or 26 depending on era - and some sleeping cars. The route itself is very scenic and features many challenging gradients and unlike the North/Kyle or West Highland Lines there is a relatively frequent service so always something moving to keep interest going.
